trying to make a "glow" effect on a scene to be composited over VSE
The only way I could do it was to produce two separate movies/images.  One for the black obects, the other for the white objects which are blurred when mixed in the compositor.

Here are the nodes...

  

One thing to check is your Render output;  Open the "Post Processing" tab, make sure PNG format is selected, and also set it's RGBA box to on. (highlited) to ensure the Alpha Layer is included in the render.

Note that a separate folder for the output png files, (one for each frame) is a good idea.

The rendered pic I got was as we see in the backdrop above, complete with an alpha layer and ready to overlay another pic or movie as required.

If you're stuck I can attach the Blend file...

ADDENDUM:
Here's how my output looks when overlaying an image or movie -



The mixing/scaling can also be done in Blender's 3D window.  I tried that and the result was the same.

The Blend file follows and will open up in the 3D window which holds instructions on how to recover the two images needed for the compositor.